Objectives Identify the social factors that contribute to reproductive failure in male common marmosets housed with the natal family. Common marmosets typically remain with their natal families into adulthood and do not reproduce while living with the family. The social mechanisms contributing to reproductive failure in male marmosets have not been identified but have been assumed to include reproductive suppression by dominant males (fathers and older brothers) and/or avoidance of breeding with closely related females (mothers and sisters). This study was designed to evaluate the roles of these two factors in male reproductive failure. Seven post-pubertal males and their fathers underwent a series of 45-minute sex tests, in which the son, the father, or the son and father together were allowed to interact freely with the mother/mate or with an unfamiliar adult female. Plasma testosterone and plasma cortisol concentrations were measured in blood samples collected from males immediately following each test and in basal blood samples collected at the same time of day on non-test days. Males engaged in very low rates of sexual behaviors when tested with their mothers. When tested with unrelated females, however, these same males exhibited significantly elevated frequencies of sexual behaviors and erections, and showed no differences from their fathers. When fathers and sons were tested together, both males tended to engage in less sexual behavior than when tested alone with the female; however, they never showed overt competition for access to the female or engaged in male-male agonism. Neither plasma cortisol levels nor plasma testosterone levels differed reliably between fathers and sons or between test conditions, but cortisol was significantly elevated above baseline levels following each sex test. These results indicate that avoidance of inbreeding, rather than dominance-related reproductive suppression imposed by the father, represents the primary social cause of reproductive failure among post-pubertal, subordinate male marmosets. Key words sexual behavior, dominance, inbreeding, testosterone, cortisol, reproductive suppression